The present invention is concerned with a method of brazing together two or more metal or alloy parts.
Brazing is a process in which a metal or alloy of lower melting range is used to join parts made from an alloy of metal of a higher melting range.
Nickel-based brazing filler alloys are well known and are used to join parts of alloys from stainless steel to much more refractory alloys such as Nimonics, Inconels and the like. These brazing filler alloys are commonly supplied as powders. They melt generally in the range from 880.degree. C. to 1200.degree. C., individual alloy compositions having solidus-liquidus ranges determined by melting point depressants such as phosphorus, boron and silicon. Combinations of boron and silicon are used in several alloys such as BS 1845 HTN1, HTN2 etc, the boron content being relatively high and in excess of 1%. These are reliably manufactured by charging to a melting furnace a formulation having the desired final composition which, when molten, is converted directly to powder by atomizing to provide appropriate melting characteristics without blending of the powder.
GB-A-1547117 discloses nickel-based brazing alloys for use in brazing or diffusion welding of metal or alloy joints, of, for example, carbon steel, nickel-based alloys, copper alloys, and stainless steel. The brazing alloys disclosed contain boron in an amount of 0.6% to 1.8% by weight. If the brazing alloys of GB-A-1547117 were to be used for the purpose of brazing thin foils of stainless steel, erosion of the relevant joint can result.
GB-A-2279363 discloses a metallic coating material for use in protecting the surface of a substrate of a copper-based alloy against high temperature corrosion and erosion. The document, however, does not teach or suggest the use of the metallic material as a brazing filler for brazing a metal or alloy part to another metal or alloy part. The high proportion (0.5% to 4.0%) of boron present, would in any case, make the material unsuitable for brazing together thin stainless steel parts, for the reasons outlined above.
The highest melting nickel-based brazing filler alloys are those which use only silicon as the melting point depressant, e.g. BS1845 HTN5, mid-range composition of 19% chromium, 10.5% silicon, balance nickel. They offer particularly useful properties in the brazing of parts for very high temperature applications. However their inherently higher brazing temperatures can impair the selection of optimum brazing conditions for a particular choice of parent metal and design of component, or increase furnace maintenance costs.
In order to overcome this problem, it is known to blend to these alloys up to 10% of a nickel-based brazing filler alloy of a lower melting range to facilitate the onset of the brazing process, especially where brazing alloy flow in to be on parent metal substrates having higher refractory surfaces. Such substrates may be, for example, iron alloys containing chromium and aluminum as used in the manufacture of metallic catalyst supports for cars, and also in honeycomb seals in gas turbines.
Conveniently this brazing filler alloy of a lower melting range contains boron as at least one of the melting point depressants present. After assisting early braze flow, the boron then diffuses into the substrates leaving the final re-melt temperature of the brazing filler alloy unaffected by any further depression in melting temperature which might otherwise be caused by the presence of boron. A convenient level of boron in the lower melting range alloy is 1% since this then results in a level of 0.1% in the resultant blend. The blend route to such a low boron content has been formerly accepted as the normal method for several reasons.
These include simple modification of a standard powder at the end of routine manufacture, in this case to BS1845 HTN5, to produce relatively small quantities of a special grade by blending. The manufacturing control required to produce a 1% boron alloy powder to within a given proportionate tolerance is also less demanding than a direct melting route producing 0.1%. In practice a blend constituent may be chemically analyzed before blending and blend ratios modified to compensate for normal tolerances of manufacture. For example, if the lower melting powder was found to contain 0.9% instead of 1% boron then the blend ratio could be modified from 10% to 11% to give the same final content of 0.1% boron. The same low melting range powder composition may be manufactured for other purposes, for example to blend with powders of other compositions, or for use as a product in its own right in the braze hardfacing process for which it was originally conceived.
For these various reasons it has become standard industry practice to manufacture brazing filler alloys with low boron contents in the typical range 0.05-0.15% by blending.
However, a disadvantage of blending two nickel based brazing filler alloy powders of different compositions is that these compositional differences from point to point will persist after melting in certain applications. In particular, when the brazing filler alloy is spread extremely thinly, which can for example involve essentially monolayers of powder particles, depending on the application. It can be shown in these circumstances that small scale statistical variations in homogeneity will lead to considerable localized variations in composition--in this case of boron concentration. Thus, the resultant film of motion filler metal immediately after melting is so thin that mixing does not occur on a scale sufficient to remove these compositional differences. This in turn leads to differences in brazing performances and therefore differences in brazing conditions and results. Sometimes the results can be very deleterious, with erosion of thin foils in the region of the brazed joint.